The grip of the tires on the ground on which they are running is, from a driver safety viewpoint, one of the most important features of a vehicle that is fitted with tires. It is also what governs the performance of the vehicle in sporty on-road driving: if its tires lose their ability to steer as a result of a lack of grip, the vehicle can no longer be steered.
Of course, a vehicle, even if intended for sporting use, has to drive under variable weather conditions. It is therefore known practice to provide the tire with means that provide good grip on dry ground and on wet ground. Notably it is possible to adapt at least part of the tread pattern to use on wet ground, for example by providing voids able to store water and/or allow water to drain away, or by increasing the number of edge corners on the tread pattern so as to cut through the film of water formed between the tread and the ground. It is also possible to vary the materials of which the tread is made, by using rubber compositions that are more particularly suited to use on wet ground and/or on dry ground. A tread having both types of rubber composition is able to provide good grip under all circumstances. An example of such a tire is given in document EP 1 308 319.
During sporty on-road driving, the tires of a vehicle experience high transverse loadings when the vehicle fitted with these tires is cornering. During the corner, the transverse loadings cause the contact patch in which each tire makes contact with the ground on which it is running, to undergo deformation that can be likened to skewing into a trapezium shape: the side of the contact patch that is furthest from the centre of the corner lengthens, whereas the side of the contact patch which is closest to the centre of the corner shortens.
The “side of the contact patch that is furthest from the centre of the corner” is the side via which, in the direction of the rate of drift of the centre of the wheel on which the tire is mounted, the elements of the tread come into contact with the ground. For this reason, it is sometimes referred to as the (transverse) leading edge. The opposite side, namely the “side of the contact patch closest to the centre of the corner” is sometimes referred to as the (transverse) trailing edge.
This “trapezoidal skew” deformation alters both the load borne by the various ribs of the tread and the contribution that each makes to the transverse loading developed by the tire. For a given load, that one of the tires of the vehicle has to bear in a given cornering rate, the ribs which have lengthened bear a greater share of the total load borne by the tire. The ribs that have shortened bear a correspondingly smaller proportion of the total load borne by the tire. For a given transverse loading, delivered by one of the tires under given cornering conditions, the result is that the most heavily loaded ribs (which in general means those on the side furthest from the centre of the corner) are those which also make the greatest contribution towards the total transverse loading.
Rubber compositions suited to use on wet ground are generally more sensitive to the very high thermal and mechanical stresses generated in the contact patch of a tire under severe cornering conditions on a dry road surface. If the tread of the tire is provided with portions made of a rubber composition that has better grip on dry ground and with portions made of rubber composition that has better grip on wet ground, it is preferable to ensure that the rubber composition that has better grip on dry ground is positioned on that side of the contact patch which is furthest away from the centre of the corner. Thus, even if the contact patch is skewed into a trapezium shape, the tire will maintain good grip on dry ground, which means to say that it will maintain a good ability to develop a high transverse loading. Furthermore, because the ground contact pressures are highest on this same side of the contact patch (which is the side furthest away from the centre of the corner), it is this part of the contact patch that generally provides the best drainage of the water wetting the road surface. As a result, this region of the tread lends itself well to the use of a rubber composition that exhibits better grip on dry ground. In other words, in this region, the tire drives as if it were driving along dry ground. There is therefore no benefit in making this part of the tread from a rubber composition that exhibits better grip on wet ground but of which the performance on dry ground is inferior to that of a rubber composition that exhibits better grip on dry ground. The “Pilot Sport 2” tire sold by Michelin is one example of a tire that has such an arrangement of rubber compositions in its tread.
Despite the good performance exhibited by this tire in terms of grip, there is still an increasing need to improve the compromise between dry grip and wet grip of tires and, more particularly, tires designed for sporty on-road driving. This is why the applicant company, in its patent applications WO 2011/076680 A1 and WO 2012/175444 A1, has proposed dividing the tread into several axis zones and distributing the rubber compositions with better wet grip and the rubber compositions with better dry grip carefully across these zones. While these solutions have made it possible to obtain a better compromise between dry grip and wet grip, they are not, however, optimal in terms of grip on wet ground or of rolling resistance.